Cellular mobile communication systems have been developed for the purpose of providing communication while ensuring the mobility of the user and have reached a stage to provide high-speed data communication services as well as voice communication services thanks to the rapid development of technology.
Handover in the cellular mobile communication systems refers to the transfer of a call between base stations to provide seamless communication to the mobile terminal. In global system for mobile communications (GSM), an example of cellular mobile communication system, inter-cell handover, among others, is the most frequent handover for the mobile terminal.
Upon inter-cell handover, when the terminal moves, a voice disconnection may occur as long as a handover latency due to signaling between the base station and the terminal, or a call drop may take place in case the delay become longer.
Further, GSM system is a frequency division duplex (FDD) system that uses different frequencies for downlink and uplink. The GSM system uses Frequency division multiple access (FDMA) and time division multiple access (TDMA), in combination, for separating users.
In general, the GSM system allocates different frequencies so that adjacent cells use the different frequencies in a fixed manner. The GSM system adopts a scheme in which the cells using different frequencies are grouped, and the frequency reuse applies to each group of cells. As information for representing a frequency reuse scheme, a frequency reuse factor (FRF) is used. For example, in the frequency reuse factor (FRF) represented by X/Y, X is a factor representing the number of cells (or sites) within a unit group, and Y is a factor representing the number of sectors in the unit group.
FIG. 1 is a view illustrating a frequency arrangement method upon a frequency reuse operation of a typical GSM system.
The frequency reuse scheme for a cell operation in a mobile communication network of the GSM system pre-allocates a frequency to be used in the sector and/or cell as shown in FIG. 1 without overlap to generate a reuse pattern in the group unit and repeatedly applies the reuse pattern to the overall network.
FIG. 1(a) illustrates that non-overlapping frequencies, respectively, are allocated to three sectors 101, 103, and 105 constituting a cell 100. The different patterns respectively marked in the sectors of FIG. 1 mean the allocation of different frequencies. In this case, the frequency reuse factor (FRF) is ⅓. In FIG. 1(a), the frequency reuse distance, which is the distance 107 between different sectors or cells reusing the same frequency is about 3.46 times the sector radius (R) 109.
FIG. 1(b) illustrates that non-overlapping frequencies, respectively, are allocated to a total of six sectors 101, 103, 105, 111, 113, and 115 constituting two cells 100 and 110. In this case, the frequency reuse ratio is 2/6. Also in FIG. 1(b), it can be seen that the frequency reuse distance 117 is about 3.46 times the sector radius (R) 109.
FIG. 1(c) illustrates that non-overlapping frequencies, respectively, are allocated to a total of nine sectors 101, 103, 105, 111, 113, 115, 121, 123, and 125 constituting three cells 100, 110, and 120. In this case, the frequency reuse ratio is 3/9. In FIG. 1(c), the frequency reuse distance 127 is about 5.19 times the sector radius (R) 109.
That is, in case the frequency reuse repeatedly applies to the entire network with each unit group of three cells containing the nine sectors, the frequency reuse distance may lengthen, that is, interference from adjacent cells or adjacent sectors is likely to be lowered.
Typically, broadcast control channel (BCCH) carriers to which frequency hopping is not applied are operated as FRF 4/12, and traffic channel (TCH) to which frequency hopping applies is operated as FRF 3/9.
However, such frequency reuse scheme has the limitation that although additional frequency resources are required for call processing due to concentration of voice calls in a particular cell, other frequencies except for ones allocated to the cell cannot be used. That is, the cellular system using the conventional frequency reuse operating scheme needs to be enhanced in terms of frequency efficiency.